Git: Limit size of git blame output when using omit-date

* Do not use setPlainTextData on VCSBaseEditor only as that one implements
  the size limits on VCS output.

* Rework removal of date data from lines to limit copying

Change-Id: Ic1f507eb7bdabdb82b4a37a3d3e6f9381775b680
Reviewed-on: http://codereview.qt.nokia.com/780


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>